Air conduction (AC) hearing aids are generally used in the rehabilitation of patients with a hearing impairment. However, for certain ear canal and middle ear disorders such as congenital malformations, chronic ear infections, draining ears, and eczema in the ear canal, etc., AC hearing aids cannot be used or are insufficient. In such cases, a conventional bone conduction (BC) hearing aid may be provided as an alternative. Bone conduction is a mechanism for delivering sound to the cochlea by sending vibrations through the skull rather than the eardrum and middle ear as in ordinary air conduction hearing.
Sound is transduced into neural impulses at the inner hair cells of the cochlea. Thus in order to achieve hearing, an actuator must have a means for moving these hair cells. In ordinary air-conducted hearing, pressure oscillations in air drive the motion of the tympanic membrane which is connected to the oval window of the cochlea through the middle ear ossicles. The stapes footplate pushes the oval window in and out, driving fluid through the cochlea. The resulting fluid pressure shears the basilar membrane to which the hair cells are attached, and their motion opens ion channels that trigger neural impulses. In BC hearing aids; when the skull vibrates, a variety of inertial and elastic effects transmit some fraction of those vibrations to the cochlear fluids and thence to the hair cells.
In a known type of bone conduction hearing devices, a vibrator is pressed against the skin of the person's head by means of a spring or an elastic headband, and which transmits the vibrations to the skull bone through the skin and the subcutaneous tissue.
Another well-known type of bone-conduction hearing devices comprises a vibrator detachably coupled via an abutment to a fixture implanted in the skull bone. The vibrator transmits the vibrations to the skull bone through the fixture. Yet another type of bone conduction hearing devices include a vibrator that is surgically implanted and affixed to the skull using screws. The vibrator transmits the vibrations to the skull bone through the screw. In all these implementations, the skull vibrations result in motion of the fluid of the cochlea, thereby stimulating the cochlear hair cells and causing the perception of sound in the recipient of the bone conduction hearing device.
For bone-conduction hearing devices, a precise determination of the vibrations applied to the skull bone is needed for determining a person's bone-conduction hearing thresholds as well as for calibrating the bone conduction hearing device. Therefore, attempts have been made to develop devices and methods for determining the applied vibrational force. For example, a proposal is made where an accelerometer is attached on a counter weight of a bone conduction vibrator. The accelerometer provides an acceleration signal, representative of an acceleration of the counter mass, from which the vibrational force may be determined. The disadvantages with such proposal include that only one specific device may be measured and incorporating the accelerometer requires space in the transducer because access to the counterweight is required.
Therefore, it is of interest to have a measurement device that is capable of determining applied vibrational force produced at the skull by a bone-conduction device. Such detection may form basis for calibrating and/or operating the bone-conduction device.